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Best Physics > Electrons, Waves and Photons > Flashcards

Electrons, Waves and Photons Flashcards

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1
Q

State Ohm’s Law

A

Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it (V=IR)

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2
Q

Electromotive force units

A

J C^-1

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3
Q

Resistance units

A

V A^-1

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4
Q

Energy units

A

V C

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5
Q

Charge units

A

A s

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6
Q

Define potential difference

A

energy per unit charge

transferred from electrical to other forms

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7
Q

What is meant by internal resistance.

A

-(some) energy is transferred into thermal energy /lost as heat in (driving charge through) the battery. It behaves as if it has an (internal) resistance
or
-there is a voltage drop across/decrease in voltage from the battery (when a current is drawn from it)

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8
Q

Define resistance.

A

-p.d./voltage (across component) divided by current (in it)

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9
Q

If two sources are coherent…

A

…they have the same wavelength and frequency and a fixed phase difference between them.

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10
Q

Define Intensity

A

energy per unit time/power per unit area (perpendicular to the direction of energy transfer)

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11
Q

State two properties which distinguish EM waves from other transverse waves.

A
  • all travel at speed of light through a vacuum

- are oscillating E and B fields or are caused by accelerating charges

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12
Q

Malus’s Law

A

I= Io cos^2(feta)

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13
Q

What is a stationary wave?

A

A stationary wave is the superposition of two progressive waves with the same wavelength, moving in opposite directions.

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14
Q

How do stationary waves form?

A
  • waves are reflected
  • reflected waves interfere/superpose with the incident
  • waves to produce nodes and antinodes
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15
Q

Define work function

A

the minimum energy required for an electron to escape from metal surface

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16
Q

Explain how the work function is related to the threshold frequency.

A
  • a photon with less than the threshold frequency cannot cause electron emission
  • so work function = h (threshold frequency)
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17
Q

Define power

A

Work done/energy transfer(red) per unit time

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18
Q

Define kilowatt-hour

A
  • (a unit of) energy equal to 3.6 MJ

- 1 kW for 1 hr

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19
Q

State and explain how the current I in the circuit changes as the thermistor is heated.

A
  • R of thermistor decreases as temperature increases
  • supply V is constant/ total R is smaller
  • current increases as V = IR
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20
Q

State with a reason how the voltmeter reading varies as the light incident on the LDR increases.

A

R of LDR decreases/current in circuit increases so V increases across fixed resistor

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21
Q

Explain the term e.m.f.

A

energy transferred from source/changed from some form to electrical energy per unit charge

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22
Q

Both electrons and photons can be considered as particles. State two differences between their properties.

A
  • electrons have mass, photons have zero mass
  • electrons have charge, photons are uncharged
  • photons travel at speed of light
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23
Q

Explain what is meant by the de Broglie wavelength of an electron.

A

electron wavelength depends on its speed/momentum

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24
Q

Displacement- wave motion

A

distance moved from equilibrium of a point/particle on a wave

25
Q

Amplitude- wave motion

A

maximum displacement (caused by wave motion)

26
Q

Frequency- wave motion

A

number of wavelengths passing a point /vibrations at a point per unit time/second or produced by the wave source

27
Q

Phase difference- wave motion

A

phase difference between two points on the same wave/waves of the same frequency, how far through the cycle one point is compared to the other

28
Q

Microwaves

A

10^-4

29
Q

Ultraviolet

A

10^-8

30
Q

Gamma Rays

A

10^-12

31
Q

Infra red light

A

10^-6

32
Q

Explain why the resistance of the filament lamp is much larger at a higher voltage.

A

the resistivity/resistance of the (metal) filament increases with temperature
the larger the current in the filament the hotter it becomes

33
Q

Emf (units)

A

J C^-1

34
Q

Energy (units)

A

V C

35
Q

What is meant by internal resistance.

A
  • (some) energy is transferred into thermal energy /lost as heat in (driving charge through) the battery. It behaves as if it has an (internal) resistance
  • there is a voltage drop across/decrease in voltage from the battery (when a current is drawn from it)
36
Q

If two waves are coherent it must mean:

A

constant phase difference/relationship (between the waves) or always at π radians/180degrees
or because they are generated by the same source

37
Q

In a series circuit with two resistors, state one electrical quantity which is the same for both resistors.

A

Current

38
Q

In a parallel circuit with two resistors, state one electrical quantity which is the same for both resistors.

A

Potential difference.

39
Q

Explain why the terminal p.d. is less than the e.m.f.

A
  • any source has an internal resistance

- where energy is transferred into thermal energy / lost as heat.

40
Q

What is meant by diffraction of a wave.

A

wavefronts/paths spread out after passing through a gap around an obstacle.

41
Q

State two features of a stationary wave.

A

-energy is trapped in pockets/ where the shape or energy
-does not move along/energy is stored
-there are nodes/positions of zero amplitude
-incident wave is reflected (at the fixed end of the string) -and the reflected wave (or it) interferes/superposes with
the incident wave (to produce the stationary wave)

42
Q

Mean drift velocity

A

the average displacement/distance travelled of the electrons along the wire per second

43
Q

Name the charge carriers responsible for electric current in a metal and in an electrolyte.

A
  • Electrons in a metal

- Ion in an electrolyte

44
Q

Explain how you would determine experimentally the e.m.f and internal resistance of the charged cell. Include a circuit diagram with meters and a variable load.

A

1.cell across variable resistor R ammeter in series and voltmeter in parallel across R or cell
2.Take (set of) readings of V and I for different positions/values of the variable resistor
3.plot a graph of V against I
4.(find) y-intercept = E
5.(find) the gradient of the V against I graph which equals the internal resistance in magnitude
or 4 or 5 take one pair of values of V,I and substitute
into equation E = V + Ir to find r or E

45
Q

Photoelectric effect

A
  1. Individual photons are absorbed by individual electrons ( in the metal surface)/ one to one interaction/AW
  2. Only photon with energy above the work function energy will cause photoelectron emission/idea of threshold frequency 3.Photon energy is proportional to frequency
  3. (therefore) blue photons with higher f/shorter λ will cause photoemission but red photons will not.
  4. hf – φ = KEmax is the equation resulting from conservation of energy or resulting from the meaning of each term
  5. A wave model does not explain instantaneous emission
46
Q

The amplitude if transmitted waves is doubled, what happens to intensity and position of maxima.

A
  • intensity increases by factor of 4

- position unchanged

47
Q

The separation between the transmitters is halved, what happens to intensity and position of maxima.

A
  • intensity unchanged

- distance apart of maxima is doubled

48
Q

The phase of transmitter A is reversed so that there is now a phase difference of 180 degrees between the waves A and B, what happens to intensity and position of maxima.

A
  • intensity unchanged

- maxima move to positions of minima (and vice versa)

49
Q

Explain what energy levels are and how they can be used to explain the emission of photon from atoms.

A
  • energy levels explanation: electrons have discrete energies in atom
  • each photon produced by electron moving between levels
  • photon energy equal to energy difference between levels
  • electron loses energy/making transition in correct direction
50
Q

State the condition necessary for electrons to produce observable diffraction when passing through matter, e.g. a thin sheet of graphite in an evacuated chamber.

A
  • wavelength of electrons

- must be comparable/of the order of magnitude of the atomic spacing

51
Q

The charge carriers moving through the electrolyte to the positive terminal of the battery.

A

positive ions/ cations

52
Q

The charge carriers moving through the wires to the positive terminal of the battery.

A

electrons

53
Q

Explain why the p.d. across the motor is not the same as the e.m.f of the car battery.

A

the battery has an internal resistance

some of the emf is across the (internal) resistance (leaving a smaller p.d. across motor)

54
Q

Explain how and why the resistance of the headlamp filament varies with the current passing through it.

A
  • The current heats the filament

- The resistance/resistivity (of the metal filament) increases (with temperature).

55
Q

Kirchoff’s 1st Law and the quantity conserved.

A

(sum of/total) current into a junction equals the (sum of/total) current out
-conservation of charge

56
Q

Kirchoff’s 2nd Law and the quantity conserved.

A
  • (sum of) e.m.f.s = (sum /total of) p.d.s/sum of voltages in/around a (closed) loop (in a circuit)
  • energy is conserved
57
Q

Describe and explain the photoelectric effect.

A

-a photon is absorbed by an electron (in a metal surface);
-causing electron to be emitted (from surface).
-Energy is conserved (in the interaction).
-Only photons with energy/frequency above the work function energy/threshold frequency will cause emission –Reference to Einstein’s photoelectric energy equation
-(energy of photon) = (work function of metal) + (maximum
possible kinetic energy of emitted electron)
-work function energy is the minimum energy to release an electron from the surface
-Number of electrons emitted also depends on light intensity
-Emission is instantaneous

58
Q

Explain what is meant by the de broglie wavelength of an electron.

A

Electrons are observed to behave as waves/show wavelike properties
where the electron wavelength depends on its speed/momentum

59
Q

Continous spectrum.

A

all wavelengths are present in the radiation.

Best Physics (9 decks)

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